A device of this kind is known from DE 43 31 042 A1 which will be described in more detail below.
Machine tools are often provided with a rotary table on which the workpiece to be machined is mounted. In the case of a known gear cutting machine (cf. e.g. the Klingelnberg company prospectus "Universal-Spiralkegelrad-Walzfrasmaschinen" KNC 40/KNC 60", no. 1329/D/GF) the rotary table bears a headstock for the workpiece and is mounted on a movable slide on the machine base. The workpiece can be rotated with the workpiece headstock about a workpiece axis (usually referred to as the B axis), the rotary table can be swivelled about a workpiece swivel axis (usually referred to as the C axis) and the slide can be slid along a workpiece positioning axis (usually referred to as the Y axis).
In the case of the known gear cutting machine, these adjustable axes are CNC controlled and each is provided with its own drive (cf. the above-mentioned company prospectus, p. 6 and 7). To simplify matters, the rotary table, spindle etc. about whose axes of rotation a controlled drive movement is to take place, are simply referred to as axes of rotation. Axes of rotation with play-free drives are required in bevel gear cutting machines as well as spur gear cutting machines.
By virtue of the fact that they are able to perform modified generating movements--also non-linear movements--of the tool relative to the workpiece, these CNC controlled gear cutting machines make it possible to obtain highly specific tooth geometries.
In many cases it will be necessary to run through a reversal of the direction of rotation of individual axes of rotation during the generating process. Such "zero passages" in the gear cutting process place extremely high demands on the automatic control of the respective CNC axis of rotation and on the entire machine structure. It is necessary for the workpiece and tool to be mounted and driven free from play so that these zero passages as well as the effect of external forces or forces resulting from the work process influence the set positions and the programmed movements of workpiece and tool as little as possible.
It is impossible to obtain a drive free from play with traditional gearing systems since known tooth designs always have a certain minimum tooth clearance in connection with the reversal of rotation if they do not possess additional devices. It is true that special tooth geometries, e.g. those of bevel gears and hypoid gears, are more suitable for play-free drive than normal spur gears are (Niemann, "Maschinenelemente", volume II, P. 366, publishers: Springer Verlag 1989), however a high drive torque is necessary when flank clearance is small and the flank clearance often needs to be readjusted due to the inevitable wear that results where there is little clearance. In order to permanently ensure a play-free drive of an axis of rotation also in the case of zero passages and under the influence of operational forces, there are a number of familiar alternatives which are described e.g. in DE 43 31 042 A1.
One of these alternatives is a drive via at least one gearwheel on the axis of rotation, with the simultaneous initiation of a compensating moment. The torque acting upon the axis of rotation is superposed by a second retroactive torque which means that flank contact only ever takes place on one side. The disadvantage of this alternative is the danger of disturbing torque arising during operation and acting on the axis of rotation, which will counteract the compensating moment. This would then result in a loss of the freedom from play.
A further alternative described in DE 43 31 042 A1 is to provide a rotary table with two separate drive systems with their own source of power. The additional drive system can have a decelerating or an accelerating effect depending on the case in question, or its direction may also be reversible in order to suppress the existing drive play. The disadvantage is the low efficiency, caused by losses in conversion, transmission and slip, the complicated construction of pre-loads required to suppress the drive play and the high costs involved. In the case of the play-free rotary table drive of a machine tool as described in DE 43 31 042 A1, a multiple-stage transmission with several intermediate shafts is employed to avoid the above mentioned disadvantages. By means of axial shifting of at least one intermediate shaft, the flanks of the toothing of the intermediate shafts can be brought into contact in mutually opposed directions thus rendering the power transmission free from play. Multiple-stage transmissions such as this are not always practicable. Especially in the case of machine tools with CNC control, it is much simpler and more accurate to act upon the drive systems of the axes of rotation instead of attempting to influence the control system by way of the transmission.
It is an object of the present invention is to improve such a device in such as way that it allows the play-free rotary movement of a rotary table or the like about a machine axis in a predeterminable angular segment in a simple and repeatable manner without the above described disadvantages.
This object is achieved according to the present invention.